Control of ph in electrodeposition of polytetrafluoroethylene



y 3, 1957 E. GRAHAM, JR] 2,800,447

CONTROL OF H IN ELECTRODEPOSITION OF POLYTETRAFLUOROETHYLENE Filed Oct. 11, 1954 INVENTOR' HAROLD EDWARD GRAHAM, JR

2,89%,447 Patented July 23, 1957 CONTROL OF pH IN ELECTRODEPGHTIGN OF POLYTETRAFLUORDETHYLENE Harold Edward Graham, Jr., Parkershurg, W. Va n signor to E. I. du Pont de Nemours and Company, Witmington, Deh, a corporation of Delaware Application October 11, 1954, Serial No. 451,481

4 Claims. (Cl. 204-181) This invention relates to a method for controlling the pH of a polymeric dispersion employed as the electrolyte in an electrodeposition process, and more particularly, it relates to the use of an ion-exchange resin to control the pH of a dispersion of polytetrafluoroethylene utilized to coat objects in an electrodeposition process.

2 In copending application Serial No. 408,171, filed by G. W. Heller on February 4, 1954, there is described and claimed a method for electrodepositing a coating of polytetrafiuoroethylene onto a conductive substrate. When such a process is maintained in continuous operation for long periods of time undesirable changes occur which result in the production of non-uniform coatings of polytetrafiuoroethylene on a substrate. The undesirable changes which may occur in such a polytetrafluoroethylene dispersion include (1) the liberation of oxygen at low current densities in dispersions having a high pH, (2) the development of some disparity between the solids content of the coating and of the electrodeposition bath at a pH greater than about 8, and (3) the increase of the concentration of various electrolytes in the electrodeposition bath.

It is an object of this invention to provide a process for controlling the pH within relatively narrow limits in an electrodeposition bath employed to coat substrates with a polymeric material. It is another object of this invention to employ an ion-exchange resin to remove interfering electrolytes and other alkaline by-products from an electrodeposition bath of polytetrafiuoroethylene solids. It is still another object to provide an improved continuous process whereby reproducibly uniform coat- 4 ings of polytetrafiuoroethylene may be electrodeposited on a suitable substrate such as a wire. Still other objects will appear in the detailed description given here: inafter.

The above objects are accomplished by continuously circulating a polymeric electrodeposition bath through a bed of an ion-exchange resin which removes interfering electrolytes, such as metal ions, and maintains the pH of the bath at a level from about 6 to about 8, and preferably about 7 to 8. In a preferred embodiment of this process wherein an aqueous codispersion of polytetrafiuoroethylene and polyisobutylene is utilized as the electrolyte in an electrodeposition bath through which wire is continuously passing and being coated with a mixture of polytetrafluoroethylene and polyisobutylene by the action of electricity, it has been found that the solids content of the bath must be maintained at a constant level and that there must not be any appreciable coagulation of the solids prior to being deposited on the wire. These operational problems may be solved by continuously pumping the electrodeposition bath in a cycle from the electrodeposition bath through a bed of ion-exchange resin to reduce the pH to about 68, and thence to a reservoir wherein the depleted solids content is replenished and from there back to the electrodeposition cell where solids are continually being removed from the bath in the form of a coating on the wir In the electrodeposition process just described there is a normal buildup of hydroxyl ions at the cathode, probably caused by the electrolysis of water causing hydrogen ions to be released, which in turn combine to form hydrogen gas, and hydroxyl ions to be released in the bath. Cations are formed in the electrodeposition bath by the release of metallic ions from the metallic anode and by the presence of salts and dispersing agents in the bath. The release of these ionic materials tends to increase the pH of the bath beyond the acceptable level of about 8 and to increase the concentration of interfering electrolytes to such a level that the current efiiciency falls so low that the deposition of polymeric solids is decreased to an undesirably low rate. The presence of some hydroxyl ions appears to be preferable in that these ions impart polarity, and thereby mobility, to the polymeric particles in the dispersion. Because of the sensitive nature of the dispersed solids in electrodeposition bath, acid cannot be added directly to the bath to neutralize the hydroxyl ions. The addition of acid causes localized coagulation of the codispersed solids and the coagulated solids do not deposit on the anodic wire in the form of a smooth uniform coating. The addition of other acidic materials has proved to be of no value, for example, if the acid is diluted sufiiciently to prevent coagulation of the solids, the addition of the diluted acid carried with it so much water that the solids content of the bath falls below an acceptable leve If solid acidic materials, such as boric acid, are employed to neutralize the hydroxyl ions in the bath, these materials are found as part of the polymeric coating which forms on the anodic substrate, and as such are undesirable inclusions. When acidic gases are utilized for neutralization purposes, the gases form anions which are discharged at the anode in the form of bubbles which disrupt the uniformity of the polymeric coating. And furthermore, with any of the above means for neutralization, there is a buildup of electrolytes in the bath due to the reaction of the alkaline by-products and the added acidic material, and such buildup results in the operational difficulties which have already been mentioned.

In the ion-exchange process, however, the alkaline byproducts react with the ion-exchange resin to produce a very small quantity of water and the spent form of the ion-exchange resin. Neither of these materials interferes with the electrodeposition process. When a sufficient amount of the ion-exchange resin has been converted to its spent form, the resin is removed from the system, regenerated, buffered, and replaced in the system for reuse.

- In the attached drawing there is illustrated a simplified schematic picture of the process of this invention. In electrodeposition cell 1 there is maintained an electrodeposition bath 2 which comprises an aqueous medium containing the polymeric solids which are to form a coating on wire 3 as it passes continuously through the electrodeposition bath 2 in the indicated direction. Wire 3 is made anodic and cell 1 is made cathodic by suitable direct current electrical connections. As wire 3 enters electrodeposition bath 2 it passes through an insulating seal 13 (e. g. a pierced rubber diaphragm) which prevents bath 3 from leaking out and insulates the electrical charge on wire 3 from cell 1. As the electrodeposition continues solids are removed from bath 2 and deposited on wire 3, while at the same time,'hydrogen is formed at the cathodic cell wall 1 releasing hydroxyl ions into bath 2. The wire coated with deposited polymeric solids may then be continuously removed from the bath, sub jected to a drying step and a sintering step, by means not shown in the drawing, and recovered as a coated wire, useful in electrical applications.

Through an adjustable overflow pipe 4, the aqueous electrodeposition bath 2 is withdrawn continuously and discharged into tower 7 which is filled with particles of a solid ion-exchange resin 8. The ion-exchange resin 8 reacts with the alkaline by-products in the aqueous bath changing to non-interfering materials such as water, insolpble salts, and the like. The purified aqueous bath liquid leaving the bottom of tower 7 is transported by pump 9 into a large reservoir 14 containing liquid composition 10 to which makeup materials are added at 12 and mixed by a suitable agitator 11 to form a'cornposition at 6 which has the proper proportion of ingredients to constitute a suitable electrodeposition bath 2. The composition at 6 is conveyed continuously in the indicated direction and'discharged into cell 1 at exitS.

It is to be understood that the above description and the drawing are illustrative of one embodiment of the process of this invention, and that other similar arrangements and modifications of this process are intended to be included within thescope of this invention.

The ion-exchange resin should be of the weak carboxylic acid type which has been modified by treatment with a bufiering solution. The sulfonic acid ion-exchange resins have been found generally to be undesirable because they are too strongly acidic and consequently they cause the pH of the electrodeposition bath to be lowered to such an extent that the dispersed solids coagulate. The carboxylic acid ion-exchange resin is likewise too strongly acidic in its hydrogen form and it therefore is necessary to treat it with a buffer solution so as to replace some of the available hydrogen ions in the resin with metal ions derived from the buffer solution.

A typical ion-exchange resin which is of the carboxylic acid type and is suitable for use in this process after the described treatment with a buffer solution is Amberlite IRC-SO. This resin is a synthetic organic polymer containing substituent carboxylic acid groups, capable of effecting cation exchange. The resin is insoluble in water, but it is somewhat swelled by water. Normally, the resin is in the form of small white, opaque beads. Other similar ion-exchange resins which are operable in the process of this invention include Duolite CS-lOO, Permutit 216, Wofatit C, and Alkalex.

The above described ion-exchange resins may, if desired, be modified by treating the acid (or hydrogen) form of the resin with a buffer solution to cause the resin to assume any desired pH. A typical buffer solution may be made by mixing 3 volumes of 0.1 molal citric acid solution and 7 volumes of 0.2 molal disodium phosphate solution. The pH of such a mixture varies from about 6.4 to 6.6. By washing the acid (or hydrogen) form of the ion-exchange resin with such a buffer solution, the ion-exchange resin will have a pH of about 6.0 to 6.5. It has been found that 3 to 4 washings, each with about 0.5 liter of buffer solution per pound of ionexchange resin, is suflicient to convert the resin to a buff ered condition. Of course, by minor variations known to those skilled in the art other desired pH values may be obtained, however in general, the pH should be from about 6.0 to 7.0 for best results. If the ion-exchange resin, before treatment with the buffer solution, is not in the acid form, it may be changed to the acid form by steeping the resin in 2% to 10% sulfuric acid, or the equivalent.

. The process of this invention is admirably suited for the coating of copper wire with a smooth homogeneous coating of polytetrafluoroethylene. The aqueous electrodeposition bath in such a process is a codispersion of polytetrafluoroethylene and polyisobutylene containing about 30-50%, and preferably about 40%-50%, solids in which the ratio of polytetrafluoroethylene to polyisobutylene is about to 1. Normally the codispersion will contain about 1% to 3%, based on the weight of polytetrafluoroethylene solids, of a dispersing agent to stabilize the codispersion. The dispersing agent may be ionic, such as the'alkali metal soaps of long chain fatty acids,

or non-ionic, such as the condensation products of ethylene oxide and phenols. The preferred dispersing agent is polyethylene glycol mono-p-octylphenyl ether.

The process of this invention is applicable to other codispersions which may be used in an electrodeposition process; for example, codispersions of polytetrafiuoroethylene and butadiene/acrylic copolymers, dispersions of synthetic rubber materials such as butadiene/styrene, butadiene/acrylonitrile, and the like, dispersions of vinyl halide polymers and copolymers, natural rubber latices, dispersions of polymers and copolymers of acrylate or methacrylate esters, and codispersions formed by mixing the two or more of the above types.

The following examples serve to illustrate the advantages of employing the process of this invention in the electrodeposition of polymeric solids onto a wire substrate. Parts and percentages are by weight unless otherwise designated.

Example 1.-This example illustrates'the high current yield obtained by using an ion-exchange resin in a process for electrodepositing polytetrafluoroethylene onto a wire. Two electrodeposition baths are prepared which are identical except that one (Bath A) is buffered but not treated with an ion-exchange resin, while the other (Bath B) is treated with an ion-exchange resin. Bath A prepared by mixing the following ingredients: 35 parts of polytetrafiuoroethylene dispersion containing 41.8% polymer solids and 1% by weight of polyethylene glycol mono-p-octylphenyl ether as a stabilizer, 5.85 parts of polyisobutylene dispersion containing 50% solids, stabilized with potassium oleate as an anionic dispersing agent and made alkaline (pH 10) with sodium hydroxide, and 10 parts of a buffer (pH 6.5) prepared by mixing 0.2 molar disodium hydrogen phosphate and 0.1 molar citric acid in about equal quantities. The pH of Bath A, after mixing, was 6.7. Bath B is prepared from the same amounts of polytetrafluoroethylene dispersion and polyisobutylene dispersion, but the buffer is omitted, and in place of the buffer there is added 10 parts of distilled water. Bath B is then treated with sufiicient ion-exchange resin, Amberlite IRC-SO, to impart a pH of 6 to the bath.

The two baths are used to coat short sections of copper wire 0.064 inch in diameter. The results are summarized below. Blank spaces indicate that current yields are not measured.

Bath A Current Yield grams per ampere-second Bath B Current Yield grams per ampere-second Cell Amperage (Amperes) (no bubbling oxygen.)

The above results show that the current yield from an electrodeposition bath treated with the ion-exchange resin bath is approximately 6.7 times higher than that observed in a similar bath which is not treated with such a resin. Moreover, at high current strengths, up to 3 amperes, there was no formation of oxygen bubbles at the anode in the bath (B) treated with ion-exchange resin, while in the control bath (A) there is incipient liberation of oxygen at a current strength as low as 0.6 ampere. One practical consideration, resulting from treatment of an electrodeposition bath with an ion-exchange resin, is that there is provided a safeguard against any chance variation in current strength that would otherwise affect the uniformity and quality of the e lectrodeposited polymeric coating.

Example 2.This'example indicates the procedure of buffering the ion-exchange resin. A two inch bed of Amberlite" IRC-SO which has been regenerated to the acid form by treatment with sulfuric acid, is placed in a column similar to the one shown in the attached drawing at 7. The resin is first washed with distilled water until the wash water has a pH of about 5.7. The resin is then rinsed with 500 ml. of a buffer solution (pH 6.3) of disodium hydrogen phosphate and citric acid. The bed is then washed several times with distilled Water until the rinse water has a pH of 6.3. An electrodeposition bath is prepared by mixing the following ingredients: 40 parts of an aqueous dispersion of polytetrafluoroethylene resin (containing 42.7% polymer solids and 0.43% polyethylene glycol mono-p-octylphenyl ether) and 6.7 parts of an aqueous dispersion of polyisobutylene (containing 50% total solids). The pH of the electrodeposition bath is determined after being passed through the ion-exchange bed the indicated number of times.

pH of bath 7.8 6.3 6.1 3 6.0 4 5.9 5 5.9 6

No. of passes:

The bed is then rinsed with distilled water until a clear rinse water is obtained. The fresh codispersion is passed through the bed and the pH of the codispersion is determined after each pass. The following results indicate that the buffered ion-exchange resin continues to control the pH of the codispersion.

No. of passes pH of codispersion In a similar experiment to that described at the beginning of this example, an identical ion-exchange resin is buffered to a slightly higher pH by passing a solution (pH of 6.9) of citric acid and disodium hydrogen phosphate through the resin bed. A codispersion of polytetrafluoroethylene and polyisobutylene, similar to that described above, is passed through the resin bed and the These comparative results demonstrate that the ion-exchange resin may be buffered to any predetermined value, and that the buffered resin will, in turn, impart this pH value to the electrodeposition bath consistently over a series of passes. Furthermore, the pH of the electrodeposition bath may be set to any predetermined value by proper buffering of the ion-exchange resin.

Example 3.Copper wire (0.064 inch diameter) is coated by the process illustrated in the attached drawing. The ion-exchange resin is Amberlite IRC-50, buffered to a pH of 6.5 in the manner described in Example 2. The electrodeposition bath is a mixture of parts of an aqueous dispersion of polytetrafluoroethylene (containing 48.7% solids and 0.49% of polyethylene glycol monooctylphenyl ether) and 3.9 parts of an aqueous dispersion of polyisobutylene (containing 50% total solids).

The thickness of the polymeric coating on the Wire is about 17 mils, and the coating does not crack upon drying. After sintering the coated wire, the coating is found to have a dielectric strength in excess of 500 volts per mil of thickness. During 5 hours of continuous operation, the solids content of the bath, the pH of the bath, and the resistance of the electrodeposition cell all remain constant in value indicating highly satisfactory operating conditions.

The process of this invention is a useful step in electrodeposition processes for continuously coating wire, piping, tubing, etc., with polymeric materials. Such coatings may be desirable because of the electrical resistance, corrosion resistance, heat resistance, or other properties which may be imparted by the coating.

I claim:

1. In a process for continuously depositing dispersed particles of polytetrafluoroethylene from an aqueous dis persion thereof onto an anodic metallic substrate by an electrodeposition method, comprising passing a current of electricity through said dispersion and said anodic substrate which method normally causes an increase in the alkalinity of the electrodeposition bath and an increase in the concentration of interfering ions and thereby reduces the current efiiciency of said electrodeposition method and produces non-uniform coatings on said substrate, the improvement in said method comprising continuously removing said aqueous dispersion from the electrodeposition cell, continuously passing said dispersion into contact with a weak carboxylic acid ion-exchange resin having a pH of 6.0 to 8.0, maintaining a contact time sufficient for said dispersion to acquire a pH substantially the same as that of said ion-exchange resin, and returning said dispersion to said electrodeposition cell.

2. The process of claim 1 in which said ion-exchange resin is buffered to a pH of 6.0 to 7.0.

3. In a process for continuously coating wire with the solid particles from an aqueous codispersion containing 30% to 40% by weight of solids comprising polytetrafiuoroethylene and polyisobutylene, by impressing an electric current across an electrodeposition cell in which said wire is anodic, said codispersion is the electrodeposition bath, and said solid particles are electronegatively charged, hydroxyl ions and other alkaline materials being continuously formed in said bath and said solid particles being continuously removed as a coating on said Wire from the bath, the step of maintaining the efiiciency of said process at a constant high level comprising continuously removing said bath from the said cell and continuously passing said bath into contact with a weak carboxylic acid ion-exchange resin having a pH from 6.0 to 8.0, maintaining said codispersion in contact with said ion-exchange resin for a time suflicient to cause said bath to acquire a pH substantially the same as that of said ion-exchange resin, continuously removing the thus treated bath from contact with said ion-exchange resin, continuously adding fresh codispersion to said treated bath in an amount which 7 replaces the amount of said solid particles removed as a 7 ng on d e. d n nu u ly r tu n ng; o. ai cell a mixture of treatedbathand fresh codispersigp said mixture having a pH from 6.0 to 8.0 and"cpntaining-;30 to 50% by weight of solids comprising polytetrafiuoroethylene and polyisobutylene,

4. The process of claim 3 in which said ion-exchange resin is bufiered' tola pH of 6.0 to 7.0.

References Cited in the file of this patent UNITED STATES PATENTS 2,478,322 Robinsqn Aug. 9, 1949 8 F RE GN: PAIENIS- 691,859 GreatBr-itain May-20; 195;?

OTHER; REFERENCES Williams: Industrial andEngineering Chemistry, June 1939, vol. 31,jNo. 6, pages 725, 726 a11dj271 Nachod: Ion Exchange 1949), pages2 77 and 278. Kemp: J.,E16CiI O Chem. S0c,, vol; 73; May 2', 1938, 10 pp. 497-502; 

1. IN A PROCESS FOR CONTINUOUSLY DEPOSITING DISPERSED PARTICLES OF POLYTETRAFLUOPROETHYLENE FROM AN AQUEOUS DISPERSION THEREOF ONTO AN ANODIC METALLIC SUBSTRATE BY AN ELECTRODEPOSITION METHOD, COMPRISING A CURRENT OF ELECTRICITY THROUGH SAID DISPERSION AND SAID ANODIC SUBSTRATE WHICH METHOD NORMALLY CAUSES AN INCREASE IN THE ALKALINITY OF ELECTRODESPOSITION BATH AND AN INCREASE IN THE CONCENTRATION OF INTERFERING IONS AND THEREBY REDUCES THE CURRENT EFFICIENCY OF SAID ELECTRODEPOSITION METHOD AND PRODUCES NON-UNIFORM COATING ON SAID SUBSTRATE, THE IMPROVEMENT IN SAID METHOD COMPRISING CONTINUOUSLY REMOVING SAID AQUEOUS DISPERSION FROM THE ELECTRODEPOSITION CELL, CONTINUOUSLY PASSING SAID DISPERSION INTO CONTACT WITH A WEAK CARBOXYLIC ACID ION-EXCHANGE RESIN HAVING A PH OF 6.0 TO 8.0, MAINTAINING A CONTACT TIME SUFFICIENT FOR 